1. Field of the Invention
The present invention relates to an image forming apparatus such as a photocopier, a printer, a facsimile and the like of electrophotographic system and in particular to an image forming apparatus having a plurality of image bearing members.
2. Description of the Related Art
In recent years, coloring in image forming apparatuses such as printers, photocopiers and the like has been being advanced. As color image forming apparatus in an electrophotographic system, so-called tandem type color image forming apparatuses that sequentially dispose a plurality of photosensitive drums corresponding with colors respectively in a line to overlap respectively colored toner images formed on respective photosensitive drums onto a transferring medium to form color images are becoming widespread as a mainstream trend.
FIG. 15 is a schematic configuration diagram showing an example of a tandem-type full-color image forming apparatus in a prior art electrophotographic system.
Such an image forming apparatus comprises four image forming parts (image forming units) of an image forming part 101M that forms magenta-color images, an image forming part 101C that forms cyan-color images, an image forming part 101Y that forms yellow-color images, an image forming part 101Bk that forms black-color images and those four image forming parts are disposed in a line in a constant interval.
Respective image forming parts 101M, 101C, 101Y and 101Bk are provided with photosensitive drums 102a, 102b, 102c and 102d respectively. Surrounding charging rollers 103a, 103b, 103c and 103d, developing apparatuses 104a, 104b, 104c and 104d, transferring rollers 105a, 105b, 105c and 105d, cleaning apparatuses 106a, 106b, 106c and 106d are respectively provided, and above between the charging rollers 103a, 103b, 103c and 103d and the developing apparatuses 104a, 104b, 104c and 104d, exposure apparatuses 107a, 107b, 107c and 107d are respectively provided. Magenta toner, cyan toner, yellow toner and black toner are housed in the developing apparatuses 104a, 104b, 104c and 104d respectively.
A method of forming images in full-color onto recording material with the above described color image forming apparatus is carried out by toner images in respective colors respectively formed on respective photosensitive drums 102a, 102b, 102c and 102d being sequentially overlapped for transfer onto recording material P born on a recording material conveying belt 108. At that time, the recording material undergoes sheet feeding with a sheet feeding roller 113 so that with an adherence roller 111 is adhered onto the recording material conveying belt 108 electrostatically and is born and conveyed with the recording material conveying belt 108. Thereafter, the recording material P separated from the recording material conveying belt 108, on which a full-color toner image is fixed with a fixing nip part between a fixing roller 112a and a pressing roller 112b of the fixing apparatus 112, is discharged outside of the machine.
In addition, frequently used is an intermediate transfecting method that brings a toner image formed with respective image forming parts into multiple transfer onto an intermediate transferring belt temporarily and thereafter into collective transfer onto the recording material P as a full-color image forming method besides the above described multiplex transferring system.
Those color image forming apparatuses do not always implement multicolor printing, that is, full-color printing, but frequency of implementing mono-color printing is rather high. Especially in the recent years, color printers in anticipation of replacement to mono-color printers have been increasing more and more, and in such color printers, performance of mono-color printer tends to be regarded very important.
In the above described prior art color image forming apparatus, the case where four image forming parts (image forming units) 101M, 101C, 101Y and 101Bk operate also at the time of mono-color image forming (black mono-color printing) happened to give rise to a problem that image forming parts 101M, 101C, 101Y in color toner that does not form images also were deteriorated in photosensitive drums and developer likewise at the time of the above described color printing.
In order to solve such a problem, a system of halting developing apparatuses 104a, 104b and 104c except black (Bk) at the time of mono-color image forming, is described in Japanese Patent Application Laid-Open No. H8-194351. According to this system, deterioration of color developer due to agitation with a developing device can be prevented.
In addition a system of providing a bypass conveyance paths dedicated for outputting mono-color image is described in Japanese Patent Application Laid-Open No. H5-341617. According to this system, mono-color images can be outputted without deteriorating color image forming parts.
Moreover, frequently used is a configuration to set a recording material conveying belt or an intermediate transferring belt spaced apart from photosensitive drums 102a, 102b and 102c of color toner at the time of mono-color print image forming so as not to consume image forming parts 101M, 101C and 101Y of color toner.
For example, there nominated are a configuration of spacing transferring means of a color-toner part apart at the time of implementing multiplex transfer directly onto recording material to form a mono-color image as described in Japanese Patent Application Laid-Open No. H11-15227 and a configuration of spacing intermediate transferring means of a color-toner part at the time of forming a mono-color image in a color-image forming apparatus of a tandem type in use of an intermediate transferring belt as described in Japanese Patent Application Laid-Open No. H07-120998.
There, such a system as described in Japanese Patent Application Laid-Open No. H05-341617 will end in complicating image forming apparatuses and increasing size, and therefore, the mainstream is the system with the configuration that sets a recording material conveying belt or an intermediate transferring belt spaced apart from photosensitive drums 102a, 102b and 102 c. 
FIG. 6 shows a configuration of setting apart transferring means at the time of mono-color printing in the configuration of direct multiplex transfer onto recording material.
Moving positions of transferring rollers 105a, 105b and 105c and a recording material conveying belt 108, photosensitive drums 102a, 102b and 102c in image forming parts other than black are set apart from the recording material conveying belt 108. Since the photosensitive drum 102d and the recording material conveying belt 108 are in contact, mono-color images can be formed and deterioration in performance of image forming parts of color toner can be reduced only in use of the image forming part 101Bk to implement image forming.
However, in case of setting apart the transferring parts of color toner, at the time when mono-color printing is carried out, the recording material and intermediate transferring belt may slide on the photosensitive drums 102a, 102b and 102c, possibly giving rise to image defects at the time of full-color printing after mono-color printing.
Detailed description will come as follow.
In use of a configuration of spacing transferring means of a color-toner part apart at the time of implementing multiplex transfer directly onto recording material to form a mono-color image as described in Japanese Patent Application Laid-Open No. H11-15227, the recording material P may end in floating from the recording material conveying belt 108 and the recording material P will slide on the photosensitive drums 102a, 102b and 102c. Sliding for one printing will not result in any problems, but nevertheless, in case of carrying out mono-color printing continuously in a large quantity, sliding will take place at the same sites of the photosensitive drums 102a, 102b and 102c, and therefore image defects (scars in sliding memory and drums) will occur at the time of full-color printing after mono-color printing.
Urging adherence force onto the recording material conveying belt with electrostatic adherence means such as the adherence roller 111 and the like, chances of sliding themselves will decrease, but resistance of the recording material could go down depending on atmospheric environment and recording material for use in image forming apparatuses, but at that time, current from the adherence roller 111 might travel on the surface layer of the recording material to flow into the photosensitive drums 102a, 102b and 102c. At that time, potential of photosensitive drums 102a, 102b and 102c might be caused to change inappropriately, image defects could occur corresponding with sliding parts at the time of full-color printing after mono-color printing even with not so many cases of sliding by making it impossible to set the sliding parts of the photosensitive drums to appropriate potentials at the time of next charging.
In particular, in a configuration of arranging image forming part vertically or diagonally, the recording material P's own weight acts in such a direction to be delaminated from recording material conveyance means and therefore conveyance of the recording material is apt to get unstable.
In addition, in case of occurrence of ruffling, core set marks and bending in the intermediate transferring belt, the belt may likewise slide on the photosensitive drums so as to give rise to image defects (scars in sliding memory and drums) at the time of full-color printing after mono-color printing also in a configuration of setting intermediate transferring means at color-toner parts apart at the time of forming mono-color images in a tandem-color image forming apparatus in use of an intermediate transferring belt as described in Japanese Patent Application Laid-Open No. H07-120998.
In particular, some atmospheric environments and belts for use in image forming apparatuses could lower the resistance of the belts and consequently bias for use with transferring bias and belt cleaning means (a cleaning system taking place simultaneously with transfer, a fur brush system and the like) could flow into the photosensitive drums 102a, 102b and 102c. At that time, changing potentials of the photosensitive drums 102a, 102b and 102c inappropriately so as to disenable the sliding parts to derive appropriate potentials at the time of next charging, image defects corresponding with the sliding parts happened to occur at the time of full-color printing after mono-color printing.
In particular, in case of having taped ends of intermediate transferring belts for reinforcing intensity, ruffling is apt to occur while core sets of belts is apt to be formed in case of a state where an image forming apparatus is not used for a long period and the present phenomena are apt to take place.
Those problems tend to get better by taking large clearance between the photosensitive drums and the recording material conveying belt or the intermediate member belt when they are set apart, but due to their contact in the black parts and accompanied by miniaturization of image forming apparatuses, which has been being desired more and more in the recent years, reservation of significant clearance has become difficult.
In addition, accompanied by miniaturization, current leakage as described above is apt to occur even if the recording material and intermediate transferring belts have the same resistance value.
Here, it is not preferable from the point of view of deterioration of image forming parts due to rotation of photosensitive drums, that has been regarded as a problem at the source, to drive image forming parts such as photosensitive drums 102a, 102b and 102c and the like at a constant speed or a slightly lower speed, which may alleviate the above described problems better, in case of separation at the time of mono-color printing.